This invention relates to the epitaxial electrodeposition of an alloy which is remarkably dense, hard, ductile, reflective, and resistant to heat, corrosion, and wear, as deposited. More particularly, this invention relates to an alloy containing nickel, cobalt, and boron (Ni-Co-B), and the electrodeposition of the alloy on a substrate at low pH and moderate temperature to form a tenacious bond with the substrate.
The epitaxial deposition of a metal alloy by an electrochemical process or chemical reduction on a surface of a substrate to modify surface characteristics of the substrate for functional or decorative purposes is well known in the art. Prior art systems disclose Ni-B, Co-B, Ni-Co, Ni-Fe, Co-Fe, Ni-Co-Fe, and Ni-Co-Tl-B alloys which are deposited on substrates to make them hard and corrosion resistant. The patent literature discloses an ongoing effort to produce such coatings which are still harder and more corrosion resistant.
The electrolytic deposition processes produce substantial amounts of sludge, which foul the plating baths and require their replacement at intervals. It has been recognized that increasing the life of the plating bath is highly desirable for both economic and environmental reasons.
U.S. Pat. No. 3,045,334 to Berzin discloses a plating bath comprising nickel sulfate, ethylene diamine, sodium hydroxide, and sodium borohydride to produce a nickel-boron alloy by an electroless procedure. Berzin substitutes cobalt chloride for nickel sulfate to produce a cobalt-boron alloy. Berzin adds a sequestering agent to the bath to prevent precipitation of insoluble metal hydroxides or basic salts. The sequestering agent comprises amines or ammonia.
U.S. Pat. No. 3,297,418, to Firestone, et al, discloses electrolytic deposition of a Ni-Co-Fe alloy. The Firestone et al process uses an acidic bath comprising nickel, cobalt, and iron sulfates, soluble saccharin, sodium lauryl sulfate, sodium chloride, and boric acid. Deposition occurred at low temperatures (20.degree. C.) in the presence of a magnetic field to produce a magnetic film.
U.S. Pat. No. 3,533,922, to Simienko et al discloses electrolytic deposition of a nickel-cobalt-iron alloy on a substrate. Simienko discloses an acidic plating bath comprising nickel and cobalt sulfates, nickel chloride, and ferrous ammonium sulfate. Additives are placed in the bath to control various parameters of the bath and plating process: potassium chloride is added to control magnetic hysteresis characteristics of the film; saccharin is added to control the uniformity of the coating, and boric acid is added as a buffer. As in the Firestone, et al patent, Simienko plates the substrate under a magnetic field to produce a ferromagnetic alloy.
U.S. Pat. No. 4,036,709 to Harbulak discloses electrolytic plating of binary and ternary alloys of nickel, cobalt, and iron. As in the prior patents, the Harbulak patent discloses a bath comprising combinations of nickel, cobalt, and iron salts with boric acid.
U.S. Pat. No. 4,833,041to McComas discloses depositing on a substrate a quaternary alloy of nickel, cobalt, thallium and boron. The deposition is preferably electroless, but may be electrolytic, using a nickel anode and the substrate as the cathode, and using a fifty amp per square foot DC current. The electroless coatings comprise hard, amorphous nodular deposits of metal alloy in a somewhat softer metal alloy matrix. The mass composition of the coating has a ratio of nickel to cobalt of from about 45:1 to 4:1, the preferred compositions having a ratio of at least 5:1. The coating is heterogeneous in thickness cross-section, having higher cobalt concentrations at the interface of the coating and substrate. With heat treatment at 375.degree. F. to 750.degree. F., the nodules showed crystalline domains of metal borides dispersed in the amorphous metal alloy matrix. The heat-treated coatings are reported to have Knoop hardness values between about 1230 and 1300.
None of these prior patents discloses the production of a ternary alloy of nickel, cobalt, and boron or a plating bath which produces such an alloy. None uses a pulsed square wave current to control the plating process or a solid catalyst to promote epitaxial deposition of a Ni-Co-B alloy on a substrate. None of the systems disclose bonding the alloy to the substrate.